Published in the ACS journal Environmental Science & Technology, the study found that when converted into a vapor, some flavorings break down into toxic compounds at levels that exceed occupational safety standards.

Since electronic cigarettes were first introduced to the market in 2003, health officials have been tracking usage and studying potential health effects.

A 2015 survey by the National Center for Health Statistics reported that 3.7 percent of adults used the devices regularly and 12.6 percent had tried them at least once.

Some studies have identified the ingredients in e-liquid flavorings, but very little research has been done to determine what happens to them when they are transformed inside the device.

A growing body of research on e-cigs has shown that the heat that converts e-liquids into vapor decomposes its contents, producing aldehydes and other toxic compounds that can potentially cause health problems.

Andrey Khlystov and his team o investigated the specific role that flavorings play in these reactions.

The researchers analyzed vapors created from both unflavored and flavored e-liquids loaded into three popular types of e-cigarettes.

The tests for 12 different aldehydes showed that the amount of potentially harmful compounds varied widely across e-liquid brands and flavors.

However, the study also showed that in general, one puff of flavored vapor contained levels of aldehydes exceeding the safe thresholds for occupational exposure - set by the American Conference of Governmental Industrial Hygienists - by factors of 1.5 to 270.